Maybe I missed your other thread about this, but would help if you included the brand/model + mods done.
50% speed reduction, as in you were shooting 850 fps prior and suddenly had a drop to 425?
Have you inspected your hammer spring to make sure its not broken? Is this rifle regulated?
That is a huge loss in speed and I can imagine is only caused by a few issues. Loss of power can result from the following:
-Hammer spring / throw out of adjustment or hammer spring broken
-Regulator issues
-Major air leak between valve and barrel (tp), breach o ring/bolt o-ring, or in the valve itself.
-Restricted air way somewhere (unlikely yet remains a possibility)
-Operating rifle below suggested pressures
-Barrel majorly buggered causing pellet to jam during its travel, but you'd likely be seeing crazy fliers
-Valve stem broken, not allowing as much lift which would restrict fps immensely
I'm sure theres more but those are the most likely/common imo that I can think of off-hand.